Faunal analysis of modern (Benguela upwelling system, Namibia) and ancient (upper Cretaceous, Israel) sedimentary records rich in organic matter, biogenic silica, carbonate, and phosphate indicates that, contrary to stereotypes of upwelling systems as dead zones, macrobenthic communities are present but highly variable. Biofacies vary with distance from the upwelling core and as a function of both the supply of food to the seafloor and the oxygen demand it creates there: under a high organic supply, large-bodied chemosymbiotic bivalves dominate exaerobic and lower dysaerobic seafloors, and deposit feeders are abundant in upper dysaerobic and aerobic zones. Macrobenthic biofacies under upwelling thus contrast strongly with the overwhelmingly filter feeder–dominated biofacies encountered in the dark siliciclastic shales on which standard oxygen-restricted biofacies models have been based, and argue that, mechanistically, the low-oxygen conditions characterizing those shales reflect ordinary levels of water-column productivity and arise largely from water-column stratification. Our biofacies model requires testing in other upwelling records. However, we expect it to be robust and useful given the long evolutionary histories of the chemosymbiotic and deposit-feeding guilds, providing a new means for discriminating the relative roles of high organic flux and low ventilation in creating low-oxygen conditions at the seafloor.

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